While flat panel displays made with LCD panels and plasma display devices are now common, flat panel displays made with electroluminescent display (EL) elements are relatively new. There are both inorganic EL devices that use an inorganic compound in the luminescent device, and organic EL devices that use an organic compound in the luminescent device.
EL device features include high speed response, high contrast, and vibration resistance. EL devices have no space inside, and can therefore be used in both high pressure and low pressure environments.
Organic EL devices operate with a low drive voltage and produce a number of gradations when used in a TFT active matrix display, but these devices are susceptible to the effects of humidity and have a short service life.
Compared with organic EL devices, inorganic EL devices are easier to manufacture because the luminescent devices are made from inorganic materials. Inorganic EL devices also feature a long service life, good vibration resistance, and can be driven with AC power. However, inorganic EL devices require a high voltage to luminesce, and are therefore difficult to drive in a TFT active matrix display. Inorganic EL devices are therefore driven in a passive matrix.
Passive matrix displays have multiple scanning electrodes extending parallel to a first direction, and multiple data electrodes extending parallel to a second direction that is perpendicular to the first direction. An AC voltage is applied between a scanning electrode and data electrode pair to drive one luminescent device. The luminescent devices are sandwiched between intersecting scanning electrodes and data electrodes. A problem with this passive matrix drive method is that the overall luminescence of the display device decreases as the number of scanning electrodes increases.
Japanese Examined Patent Publication No. S54-8080 teaches doping a phosphor layer of primarily ZnS with Mn, Cr, Tb, Eu, Tm, or Yb, for example, to improve light output and improve the peak luminance, but the average luminance is less than 400 cd/m2 and this display produces fewer than 256 gradations. It is therefore unsuitable for use as a display device for televisions, for example.
When luminescent devices are used as a display device for televisions, for example, the screen will be darker than an LCD panel if the luminance of the luminescent devices is not at least 400 cd/m2. The luminance of luminescent devices used in such displays must therefore be 400 cd/m2 or more.
Furthermore, when luminescent devices are used in televisions and other such display devices, 256 gradations or more are required. In conventional passive matrix display devices using inorganic EL devices that use an inorganic compound for the luminescent device, there is a sharp change in luminance relative to the applied voltage as shown by the dotted line representing a prior art device in FIG. 4. The difference between this threshold voltage and the voltage achieving the maximum peak luminance is small, and because graduated control of light output by means of external electric field control is either very difficult or impossible, it has not been possible to develop a viable display device that can be used in televisions, for example.
An object of the present invention is therefore to provide a luminescent device that provides both sufficient brightness and enables 256-level gray scale control.